1. Field of the Invention
The present invention generally relates to image forming apparatuses and sheet transporting apparatuses. More specifically, the present invention relates to an image forming apparatus where a sheet transported to a platen guide plate is suctioned (forced) onto the platen guide plate by a suction unit and an ink droplet ejected from a recording head is attached to the surface of the sheet to form an image, and a sheet transporting apparatus.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile apparatus, a copying apparatus or a printer/fax/copier multi-function peripheral, for example, there is known an inkjet recording apparatus in which a recording head (image forming unit) including a liquid ejection head is used to eject droplets of recording liquid. While a sheet (the material is not limited to paper, and a recorded medium, a recording medium, a transfer member, or a recording paper may also be used having the same meaning) is transported, the droplets of the recording liquid (hereinafter the “ink droplet”) are attached to the sheet and image formation (recording, imaging or printing may also be used having the same meaning) is performed.
When an image is formed by an inkjet recording system, at the time of printing when ink is jetted onto a sheet and an image is formed, the planar precision of the surface of the sheet is important. In the sheet, for example, when humidity is high or the sheet is thin, a state where a sheet edge is curved (hereinafter, this state is called “curl” or “loop”) is likely to occur. When the sheet in which such a curl occurs is directly transported to a platen guide plate and printing is performed, the distance between a nozzle of a recording head and the surface of the sheet varies. Thus, the sheet may come in contact with the nozzle surface of the recording head, so that the nozzle surface of the head is soiled or the sheet itself is soiled. Besides, the landing position of the jetted ink droplet is shifted so as to influence the image quality regarding coloring, white streaks, black streaks or the like, and there is a risk that the image quality will be degraded.
As an image forming apparatus to prevent the influence due to the deformation of a sheet as stated above, for example, as disclosed in patent document 1, there is an apparatus where a paper feed motor is stopped just before a sheet reaches a print area; sheet transport is temporarily stopped; the driving of the paper feed motor is resumed after a specified standby time required to change a reverse curl state of the sheet to a forward curl state elapses; and a sub-scanning motor is driven to transport the sheet adsorbed on a transport belt to the print area.
[Patent document 1] Japanese Patent Application Publication No. 2007-45596
However, in the image forming apparatus disclosed in the patent document 1, the standby time in which the reverse curl state of the sheet is changed to the forward curl state must be changed according to conditions such as humidity and paper quality. When the standby time becomes long, a time (print time) in which the sheet passes through the print area becomes long. Thus, there is a problem that printing efficiency may be reduced.